Semiconducting structures serve as the basis for the electronics industry. In order to improve performance, methods have been developed for more and more increasing the density of circuits per unit area. However, a physical limit is being approached. It is for this reason that three-dimensional integration methods have appeared. Instead of always reducing the size of circuits, it is sufficient to stack them in 3D structures and to connect them by vertical interconnections.
The production of these types of structures requires the successive transfer of the layers of which they are made. These layers are, in point of fact, produced separately on specific substrates called “original substrates.” It is then necessary, in order to transfer them in the right direction onto their final substrate, to use a temporary substrate, also called a “sacrificial substrate.”
The steps of an example of such a transfer are shown in FIG. 1. A temporary substrate 100 is attached to the topside of a layer 4 to be transferred, and part of an original substrate 5 on which the layer 4 was formed is then removed. The layer 4 is attached to a final substrate 8, covered as the case may be with one or more other layers 7 in the case of a 3D structure. Finally, the temporary substrate 100 is demounted.
Other applications exist for temporary substrates like, for instance, for mechanically supporting a semiconductor substrate while this substrate is processed (thinning operations, interconnection and/or vias formation, deposition of functional layers . . . ). Such temporary substrates and applications are described in U.S. Patent Application Publication No. 2004/222500.
One difficulty lies in the final detachment of the temporary substrate 100. A first technique called “laser lift-off” requires the use of a transparent substrate and an adhesive material that is sensitive to laser excitation. Another technique, described in U.S. Patent Application Publication No. 2004/222500 mentioned above, proposes the use of a rough temporary support that may be detached by a final mechanical and/or chemical action.
These solutions give satisfaction but remain very slow to process: the larger the substrate, the longer the detachment time. Although for 100 mm diameter substrates the detachment time may be acceptable, this detachment time proves to be much too long for profitable industrial use for substrates having diameters of 200 mm, 300 mm, or more.
European patent application EP 1889887 discloses a temporary substrate with an adhesive layer incorporating thermally expandable microspheres, but the temporary substrate described in this document cannot be used for a processing step that requires a temperature greater than 140° C. Additionally, the nature of the adhesive layer and the uncontrolled disposition of the microspheres into the adhesive layer does not meet the requirements of providing a sufficiently high adhesion during processing and enabling the controlled detachment of the substrate after processing. This is particularly true if the processing steps are exerting high levels of stress on the attached assembly, as is the case with grinding or polishing operations.
Accordingly, improvements are needed over these prior art processes, and these are now provided by this disclosure.